1. Field
The present invention relates to disk drives for computer systems. More particularly, the present invention compensates for thermal expansion when writing spiral tracks to a disk of a disk drive.
2. Description of the Related Art
When manufacturing a disk drive, product servo sectors 20-2N are written to a disk 4 which define a plurality of radially-spaced, concentric servo tracks 6 as shown in the prior art disk format of FIG. 1. The product servo sectors are said to form servo wedges or servo spokes from the outer to inner diameter of the disk. Each product servo sector (e.g., servo sector 24) comprises a preamble 8 for synchronizing gain control and timing recovery, a sync mark 10 for synchronizing to a data field 12 comprising coarse head positioning information such as a track number, and servo bursts 14 which provide fine head positioning information. During normal operation, the servo bursts 14 are processed by the disk drive in order to maintain a head over a centerline of a target track while writing or reading data. In the past, external servo writers have been used to write the product servo sectors 20-2N to the disk surface during manufacturing. External servo writers employ extremely accurate head positioning mechanics, such as a laser interferometer, to ensure the product servo sectors 20-2N are written at the proper radial location from the outer diameter of the disk to the inner diameter of the disk. However, external servo writers are expensive and require a clean room environment so that a head positioning pin can be inserted into the head disk assembly (HDA) without contaminating the disk. Thus, external servo writers have become an expensive bottleneck in the disk drive manufacturing process.
The prior art has suggested various “self-servo” writing methods wherein the internal electronics of the disk drive are used to write the product servo sectors independent of an external servo writer. For example, U.S. Pat. No. 5,668,679 teaches a disk drive which performs a self-servo writing operation by writing a plurality of spiral tracks to the disk which are then processed to write the product servo sectors along a circular path. Each spiral track is written to the disk as a high frequency signal (with missing bits), wherein the position error signal (PES) for tracking is generated relative to time shifts in the detected location of the spiral tracks. In addition, the '679 patent generates a servo write clock by synchronizing a phase-locked loop (PLL) to the missing bits in the spiral tracks, wherein the servo write clock is used to write the product servo sectors at the appropriate circular location as well as at the appropriate frequency.
In order to write the product servo sectors at the proper disk locations during the self-servo writing operation, it is important for the spiral tracks to be written to the disk with a uniform spacing between the spiral tracks. If an external spiral servo writer is used to write the spiral tracks individually, various components of the external spiral servo writer as well as the disk itself may experience thermal expansion. In addition, the effect of thermal expansion may not be uniform around the circumference of the disk (relative to the head). If the spiral tracks are written without compensating for this thermal expansion, the resulting non-uniform spacing between the spiral tracks may degrade and/or render the product servo sectors inoperable.
There is, therefore, a need to compensate for thermal expansion when writing spiral tracks to a disk of a disk drive.